As mobile communication terminals such as personal digital assistants (PDAs), smart phones, cellular phones and music phones are widely used and functions thereof are diversified, a utility range gradually expands.
Generally, as a means for displaying an image, a liquid crystal display (LCD) device is mounted on a mobile communication terminal. Electro-static discharge (ESD) is generated while a product mounting an LCD device thereon is in use. When the ESD is generated, internal devices of a mobile communication terminal are damaged and may not properly operate.
FIG. 1 is a graph illustrating a general waveform of current when the ESD is generated.
The ESD means a phenomenon that an electric charge moves between two objects. Referring to FIG. 1, the ESD has a characteristic that a current rapidly rises for less than 1 ns and then is discharged for several tens of ns. An x-axis of a graph illustrated in FIG. 1 is a time (unit: ns) for which a current A is generated, and a y-axis is a relative amount of a current A.
Any exposed portion (e.g., a portion having a structure allowing easy discharge of an electric charge such as a lightning rod) of a mobile communication terminal may serve as a discharge path for the ESD. Also, an electric charge caused by the ESD may be introduced into an inside of a mobile communication terminal, which is not fatal to a human body, though, but damages an electronic product during a manufacturing process of the product or in a field area where the product is actually used, resulting in disorder of the mobile communication terminal.
FIG. 2 is a view illustrating an example where a general mobile communication terminal normally operates and an example where a general mobile communication terminal does not normally operate due to ESD.
Referring to (a) of FIG. 2, the mobile communication terminal 10a normally operates before the ESD is generated. A normal screen 12a is displayed as illustrated.
On the other hand, referring to (b) of FIG. 2, the mobile communication terminal 10b does not normally operate when the ESD is generated. Only an initial screen 12b is displayed instead of the normal screen as illustrated. The shown abnormal operation of the screen 12b is generally called “white screen state”.
The “white screen state” means that operations of respective parts of an LCD device are suspended as ESD is generated, and the LCD device is switched into a state where only power is applied. At this point, a user should initialize power in order to restore the mobile communication terminal to its normal state.
However, besides the “white screen state” when internal elements of an LCD device are damaged by the ESD, the mobile communication terminal may not operate eternally.
FIG. 3 is a view illustrating an example where an ESD endurance test is performed using an ESD experiment apparatus, and FIG. 4 is a view illustrating an example where kinds of a discharge tip used for an ESD gun.
As an example of the ESD test, charges existing on a portion of a display window of a mobile communication terminal 10 are artificially discharged using the ESD gun 20 having the discharge tip 22 with a predetermined voltage as illustrated in FIG. 3. Here, functions of the mobile communication terminal 10 are checked. The artificial discharging is performed using an air type ESD test and a contact type ESD test. Referring to FIG. 4, different kinds of discharge tips 22a and 22b are used depending on whether the ESD test is the contact type ESD ((a) of FIG. 4) test or the air type ESD test((b) of FIG. 4).
At this point, the discharge tip 22a or 22b is perpendicularly positioned on a case surface of the mobile communication terminal 10, and a resistor having resistance of about 940 kΩ is connected between the mobile communication terminal 10 and the ground. Also, the mobile communication terminal 10 should be located on a discharge board. In the case of the air type ESD test, the discharge tip 22b is closely located on a test point and shot-discharged one time. In the case of the contact type ESD test, the discharge tip 22a contacts a test point and shot-discharged one time.
Currently, when the above-described endurance test is performed on LCD device products by an N company and a T company, which are widely used among the public, the white screen state is generated at a voltage of about 8 kV in the case of the air type ESD and a voltage of about 5 kV in the case of the contact type ESD.
According to such test results, products are under a great influence of the ESD during a manufacturing process of the products or in a field area where the products are actually used. Therefore, development of an LCD device having enhanced ESD durability is highly required.